Algal growth in swimming pools leads, first, to a coating of slime on the walls and bottom, then, to an unpleasant green discolouration of the water and, finally, to the proliferation of micro-organisms and other aquatic life forms, some of which may be pathogenic for humans. The conventional ways of controlling such growth are chemical and mechanical in nature. The chemical approach is to add toxins, such as chlorine and algacides, to the pool water to kill the algae. The mechanical approach is to scrub the bottom and sides of the pool, by hand or with a moving suction-head, to dislodge the algae and to pump the pool water through a filter to remove the free-floating or dislodged material. Almost all pool owners use both chemical and mechanical treatments, though many have reservations about the wisdom of using algacides in their pools, from the standpoint of the health of swimmers.
Pool owners generally recognise that the effort and expense needed to achieve a given level of control over algal growth increases with the age of a pool and with pool usage. It is also generally appreciated that the water of well-used pools tends to develop an unpleasant acrid odour and to irritate the eyes and skin of swimmers, it being known that this is due to relatively high concentrations of chloramines in such pools. The recommended treatment is an extended period of super-chlorination (during which the pool cannot be used), but the chloramine--and algal growth--problems soon return after such a treatment. The situation is essentially the same for fresh and salt pools.
In contrast to current practice, the present invention is based upon the simple idea that algae in swimming pools may be starved rather than poisoned in order to control their growth. This is done by removing from the pool water one or more dissolved nutrients essential for algal growth, such as those containing phosphorus and nitrogen.
The importance of phosphate as a critical algal nutrient in lakes and streams was charted in a series of definitive studies reported by Vollenweider in 1968 (OECD DAS/CSI/68.27). In the 1970s the US Environmental Protection Agency (EPA) commissioned a number of studies of the feasibility of nutrient inactivation in lakes [eg, EPA-660/3-74-032, October 1974--NTIS PB-239 969] by precipitation with the soluble salts of Al, La, Zr and Ti. In one reported experiment, LaCl.sub.3 was formed by dissolving La.sub.2 (CO.sub.3).sub.2 in HCl and added to water to precipitate phosphate as LaPO.sub.4. There was some concern, however, about the toxicity of unreacted LaCl.sub.3 in fish and other aquatic organisms. German patent No 2,520,210 to Altmann (1975) discloses the use of soluble rare earth salts, specifically La(Na.sub.3).sub.3, to precipitate phosphate from swimming pool water, the liquid reagent being added to the bulk of the pool water. He suggests that the resultant fine suspension (which is quasi-stable and turns the pool milky) can be removed by filtration or by using conventional flocculants. But the sub-micron particles which form the suspension pass through most pool filters and require unacceptably large amounts of flocculant to bring down, resulting in excessive filter-blockages.
The general affinity of La and Zr for dissolved phosphates is well known and exploited in other arts. For example, La is used to tag or remove phosphate ions in cellular biology, particulate hydrous ZrO.sub.2 is used to absorb phosphates from body fluids in kidney dialysis (eg, U.S. Pat. No. 3,850,835 to CCl Life Systems) and Japanese patent application No 296776 by Ebara discloses the use of particulate ZrO.sub.2 and TiO.sub.2 to remove phosphates from acidified waste liquors containing high concentrations of Al and Fe.